In Joda-Time, set DateTime to start of month

Midnight at the start of the first day of the current month is given by:

// first midnight in this month
DateMidnight first = new DateMidnight().withDayOfMonth(1);

// last midnight in this month
DateMidnight last = first.plusMonths(1).minusDays(1);

If starting from a java.util.Date, a different DateMidnight constructor is used:

// first midnight in java.util.Date's month
DateMidnight first = new DateMidnight( date ).withDayOfMonth(1);

Joda Time java doc – https://www.joda.org/joda-time/apidocs/overview-summary.html